In the production of spectrally sensitized silver halide emulsions, sensitizing dyes are usually added to emulsions after the formation of silver halide grains. To the contrary, methods of adding sensitizing dyes to emulsions before completion of the formation of silver halide grains are disclosed in U.S. Pat. Nos. 2,735,766, 3,628,960, 4,183,756 and 4,225,666. In particular, U.S. Pat. Nos. 4,183,756 and 4,225,666 describe that the addition of sensitizing dyes to emulsions before completion of the formation of silver halide grains brings about the advantage of (1) an increase in photographic sensitivity, (2) an increase in stability with respect to the passage of time, (3) prevention of desorption of dyes and color remaining, (4) prevention of variation of light absorption characteristic of dyes, and (5) prevention of change of the crystal habit of the silver halide grains.
Many currently employed sensitizing dyes are substantially water insoluble. Such water-insoluble dyes are generally dissolved in an organic solvent, such as methanol, ethanol, methyl cellosolve, pyridine, etc., and added to an emulsion before coating. To the contrary, it is difficult to add an organic solvent solution of the substantially water-insoluble sensitizing dye to an emulsion before the formation of silver halide grains is completed, for the reasons set forth below. First, since formation of silver halide grains is usually carried out at high temperatures (e.g., 50.degree. to 80.degree. C.), addition of a large quantity of an organic solvent solution of a dye during the formation of grains induces evaporation of a large amount of the organic solvent, which is not only harmful to workers, but also creates a danger of explosion. On the other hand, since an emulsion before coating can be dissolved at a low temperature (e.g., 40.degree. C.), addition of an organic solvent solution of a dye to an emulsion before coating involves little evaporation of the organic solvent. Second, addition of an organic solvent to an emulsion is likely to give rise to problems ascribed to formation of condensates of hydrophilic colloids.